The fundamental operation of a test instrument is to capture the response of a device under test to a known stimulus. In general, test instruments have the capability of applying a precisely controlled electrical signal or stimulus to a desired circuit in a device under test. With the development of increasingly faster integrated circuits there is a corresponding increase in the demands made upon test instruments used for quality control or evaluation of the integrated circuits, sub-assemblies and completed devices in which the circuits are used.
Typically, test instruments require accurate calibration of a controllable delay element to generate a known stimulus. Some test instruments undergo a factory calibration procedure, during which the test instrument is the device under test. A stimulus is applied and the result of the stimulus is recorded using an oscilloscope or other suitable test equipment. Results, recorded during the calibration are applied to software, which translates the stimulus and result information to a function or table that describes operation of the controllable delay element and related circuit components. The function or table may account for changes in frequency, temperature or other factors that effect performance of the controllable delay element. The function or table is often stored in a memory with the test instrument.
Delay elements and related circuit components are known to vary in response due to manufacturing variation, operating frequency and temperature, among other factors. A factory calibration is somewhat limited in that temperature or aging instability of one or more components of the controllable delay element and related circuit components cannot be corrected. As a result, undesired inaccuracies may be unknowingly introduced by the controllable delay element.
U.S. Pat. No. 7,062,733 describes circuits and methods for delay line calibration. The methods compare sub-sampled signals to determine time delay, calibration of delay elements, and other precise time domain measurements based on properties of aliased signals produced by the sub-sampling. The time delay is determined and averaged over a measurement window and then scaled to determine an amount of delay in the delayed signal. While the circuits and methods for delay line calibration purport to remove the necessity of performing a factory calibration, the sub-sampling techniques are complex and sensitive to short term temperature stability, require stable controllable delay elements and accurate monitoring of system temperature.
Therefore, it would be desirable to provide a low cost, reliable and integrated delay element calibration solution that can perform across a range of devices and environmental conditions.